This invention relates generally to heat pumps and, more particularly, to a method and apparatus for determining when a defrost procedure should be initiated.
An inherent characteristic of an air source heat pump operating in the heating mode is the tendancy to have frost formed on the evaporator coil, with the frost buildup causing a decreasing system efficiency. Accordingly, it is common practice to periodically initiate a defrost cycle by reversing the system (i.e. operating in the cooling mode) for a period of time. Ideally, one would like a defrost system to turn on only when the frost buildup has reached a predetermined level or when the system efficiency has decreased a certain percentage, and then to remain in the defrost mode only for so long as necessary to affect complete defrost. Various control methods and apparatus have therefore have been devised for that purpose.
Known methods of determing the degree of frost buildup on the coil include: using photo-optical techniques; sensing the speed of the fan blade; and measuring the difference in the refrigerant pressure between the inside and the outside coil all of which have certain disadvantages. Another approach that is employed in a so called "demand defrost" system is that of sensing the temperature differences between the coil and the ambient air and when that difference reaches a predetermined level, initiating the defrost cycle. It will be recognized that when this approach, the use of two sensors is required. This, in turn, complicates the solution because of the need to calibrate the two sensors in order to obtain accurate temperature measurements. That is, the thermistors presently available have inherent differences such that when a pair are used, it is necessary to conduct a calibration process for each individual system, which can be time consuming and expensive. Although there are other types of sensors available which are reasonably accurate without calibration, their use in an adaptive defrost system is not economically justifiable.
It is therefore an object of the present invention to provide an improved adaptive defrost system.
Another object of the present invention is the provision in a heat pump adaptive defrost system for maximizing the efficiency over a complete cycle of operation.
Yet another object of the present invention is the provision in an adaptive defrost system for measuring frost buildup on a coil without the use of expensive temperature sensors or calibration techniques.
Still another object of the present invention is the provision for an adaptive defrost system which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.